The invention relates to arrangements and methods in the field of radio communications, and is particularly concerned with the transmission and reception of radio signals that are encumbered with fading.
In order for radio communication between a transmitter and a receiver to function, a basic requisite is that the amplitude of the varying electromagnetic field (the signal) generated by the transmitter is sufficiently strong in relation to the natural signal noise level in the receiver for the receiver to detect the signal. It is possible to distinguish between three different phenomena that weaken the radio signal on its path from transmitter to receiver.
Firstly, the signal may be weakened due to the distance between the transmitter and receiver; the longer the distance, the weaker the signal received by the receiver. Secondly, the signal may also be weakened by shadowing objects in the path of the signal, such as natural formations and building structures. Thirdly, the signal transmitted by the transmitter may be weakened as it approaches the receiver by reflections against a number of reflecting surfaces. Depending on the difference in total distance between transmitter and receiver with respect to the various reflected signals, these signals will coact (interfere) more or less destructively and, in the worst case, cancel out each other and therewith give rise to a minimum in the interference pattern that has manifested at the receiver site. For instance, when considering the case of two interfering signals whose wavelength difference is one-half the signal wavelength, the signal will be completely extinguished. If the surroundings of the transmitter and the receiver change so as to cause the reflection conditions to change constantly, for instance because the receiver is mobile, this interference will be observed on the receiver side as so-called fading moments. Alternatively, the problem can be described by saying that the receiver is located in a fading minimum. Depending on the signal wavelength in relation to the rate at which the surroundings change, the state of these fading minima will vary spatially and also in time. For instance, fading moments occur with a typical length of one tenth of a second when the wavelength is 0.33 m (corresponding to a frequency of 900 MHz) and the relative speed between transmitter and receiver is a typical walking speed of some km/h. If the receiver remains stationary when it has reached a fading minimum, the received signal may fail to appear for a much longer time.
The problem of fading in radio reception has earlier been solved with the aid of so-called diversity arrangements. In principle, this solution has involved connecting to a radio receiver two or more antennas whose mutual position has caused the signal environment to be different for respective antennas. This is utilized in the diversity receivers by utilizing the strongest signal from one antenna, or by using a combination of the signals for more than one of said antennas.
JP 59-72831 proposes a solution to the fading problem. There is described a diversity radio receiver which includes two separate receiver antenna that are connected to a receiver unit which includes a diversity function. The signal strengths from the two antennas are compared continuously and the antenna that receives the strongest signal at that moment in time delivers the signal to the actual receiver unit.
U.S. Pat. No. 5,361,404 describes a diversity receiver which is equipped with at least two antennas. With the intention of reducing the fading effect, the signals from these antennas are combined with the aid of a control unit. The control unit controls amplification and phase-shifting of the signals from the different antennas, wherein the optimal signal may be either the signal from one of the antennas or a weighted sum of the signals from several of the antennas.
U.S. Pat. No. 5,191,598 proposes a method of reducing the effects of fading in a radio communications system. In a receiver having at least two receiver antennas, the transmission functions of respective channels in which the antennas are included are assessed with the aid of a signal processing unit. These assessments of the transmission functions are then used in a Viterbi algorithm to recreate the ideal input signal.
The drawbacks with solutions of this type is that they include complicated arrangements with combinations of hardware and software that measure and compare the signal strength of two or more antennas.